1. Field of the Invention
The present invention relates to a drive train for a hybrid electric vehicle which combines a fuel combustion engine and a electric motor/generator as power sources to propel a motor vehicle in highly efficient driving performance.
2. Description of the Related Art
A drive train for a hybrid electric vehicle of this kind is disclosed in Japanese patent laying-open publication No. 2003-32808. This drive train has an engine, a first motor/generator, a second motor/generator, a differential device consisting of planetary gear sets connected with the engine, the first and second motor/generators, and a final drive. When vehicle starting, the drive train is shifted to a large driving force running mode, because large driving force is necessary for starting a motor vehicle. This running mode is set so that an amount of electricity generated by the first motor/generator at a low vehicle speed, for example 0 Km/h to about 30 Km/h, is larger than an amount of electricity supplied to the second motor/generator, because the first and second motor/generators are operated so as to absorb power corresponding to output power outputted from the engine when the large driving force for starting the vehicle is needed.
The above known conventional drive train, however, encounters a problem that when a battery is full-charged, surplus power generated by the first motor/generator is wasted into heat energy, resulting in degradation of fuel efficiency. Furthermore, this heat loss requires measures for cooling the first motor/generator.
It is, therefore, an object of the present invention to provide a drive train for a hybrid electric vehicle which overcomes the foregoing drawbacks and can efficiently use surplus power caused by a power balance between a first motor/generator and a second motor/generator for increasing driving force.